1. Field of the Invention
This invention relates generally to subscriber line interface circuits, and, more particularly, to implementing protection in subscriber line interface circuits.
2. Description of the Related Art
Conventional telephones receive signals from the network over a loop formed using two wires that are referred to as the tip line and the ring line. The subscriber line interface circuit provides an interface between the two wire (twisted-pair) loop and the four wire ground referenced low-voltage switch environment supported by the network. The subscriber line interface circuit is vulnerable to power surges such as those caused by lightning strikes because the subscriber line interface circuit is electrically coupled to the phone loop, which is exposed to the environment. The subscriber line interface circuit must therefore be able to withstand the surges of energy that are produced by lightning strikes and other disturbances. Conventional subscriber line interface circuits include a separate protection circuit coupled to each tip lead and a separate protection circuit couple to each ring lead.
FIG. 1 conceptually illustrates a conventional subscriber line interface circuit 100 that includes separate protection circuits 105(1-2) coupled to the tip lead 110 and the ring lead 115, respectively. In the interest of clarity, the indices (1-2) may be dropped when referring to the protection circuits 105 collectively. This convention may also be used with other elements shown in the drawings and referenced by a numeral and one or more distinguishing indices. Each protection circuit 105 includes a silicon-controlled rectifier or thyristor 120 coupled in series with a diode 125, as well as resistors 130 that are coupled in parallel with the thyristor 120 and the diode 125. The gates of the thyristors 120 are coupled to a control circuit (TA) 135, which is coupled to a battery that is housed at the telephone office and which provides a voltage (VBAT) that is typically set at approximately −48 V. The gates of the thyristors 120 are also coupled to the diode 125 via resistors 130(2, 4). In operation, the protection circuits 105 protect the subscriber line interface circuit 100 by dumping energy from power surges to ground via the thyristor(s) 120 and/or the diode(s) 125.
Utilizing a different protection circuit 105 for the tip lead 110 and the ring lead 115 has a number of drawbacks. For example, the area of the wafer that is consumed by the protection circuits 105 increases in proportion to the number of tips/ring leads 110, 115 when the protection circuits 105 are formed on silicon wafers or substrates. For another example, I/O pads or pins are required to test each of the protection circuits 105 formed on a die and so using a larger number of protection circuits 105 requires a larger number of tests I/O pads or pins. Consequently, conventional protection circuits 105 are not integrated with the subscriber line interface chip.